Douglas Carswell Said Conservative Leadership Not Serious About Changing Britain's Relationship With Europe

LONDON—Douglas Carswell, a freethinking and outspoken euroskeptic lawmaker from Prime Minister David Cameron's party, said on Thursday that he was defecting to the U.K. Independence Party and forcing a by-election because the Conservative leadership wasn't serious about changing Britain's relationship with Europe.

The surprise defection is a blow to Mr. Cameron with just nine months to go before May's general election. The defection comes despite Mr. Cameron's concessions to euroskeptic lawmakers in his party like Mr....